Cemented carbide materials are well known for their unique combination of properties of hardness, strength, and wear resistance and have accordingly found extensive use in mining tool bits, metal cutting and boring tools, metal drawing dies, wear resistant machine parts, and the like. See for example, McKenna, U.S. Pat. No. 2,113,353.
It is known that the wear resistance of cemented carbide materials may be enhanced by the application of thin coatings of a highly wear resistant material such as titanium carbide or aluminum oxide. See for example, Lambert et al., U.S. Pat. No. 4,357,382 and Tobioka et al., U.S. Pat. No. 4,150,195. The most important factors influencing wear during machining are:
1. Mechanical abrasion; PA1 2. Diffusion and reaction between the material machined and the cutting tip at higher cutting temperatures; and PA1 3. Thermal cracking and chipping.
Economic pressures for higher productivity in machining applications are placing increasing demands upon the performance of cutting tool materials. To achieve high productivity in machining, a tool must be able to cut at high speeds. At cutting speeds exceeding 1500 surface feet per minute (sfpm), the high temperature strength and chemical inertness of a cutting tool material become more and more important. The usefulness of cemented carbide cutting tool materials (the predominant material used in cutting tools today) has been extended to applications requiring cutting speeds of about 1500 sfpm by coating such tools with aluminum oxide. For cutting speeds in excess of 1500 sfpm, cemented carbide tools encounter problems associated with loss of strength and tool nose deformation, which affect dimensional tolerance in the workpiece and contribute a shorter tool life.
Indexable cutting tool inserts coated with thin surface layers ocf hard materials up to a few micrometers thick are also established in the market and their importance is steadily increasing. Such inserts are principally used for turning and milling of steel and cast iron.
Two main advantages arise from the use of coated inserts: the increase in tool life, which can be several times that of uncoated inserts, and the possibility of using increased cutting speeds and thereby reducing machining time.
Cutting tools are typically coated with either single or multiple layers. Most multiple layer tools are coated first with TiC which chemically interacts with the substrate to promote adherence and subsequently with TiN for crater wear resistance. Some flank wear resistance is sacrificed with the softer outer layer to gain improved crater wear resistance. A first or single layer of TiN has generally proven unsuccessful because the higher thermal expansion of TiN compared to TiC, 9.4.times.10.sup.-6 .degree.C..sup.-1 and 7.4.times.10.sup.-6 .degree. C..sup.-1 respectively, creates much higher residual stresses in the TiN coating when the coated tools are cooled from the CVD processing temperature (ca. 1000.degree. C.) and the coating tends to be poorly adhered to the substrate.
The first coated inserts, which were introduced about fifteen years ago, were coated with TiC. After this, a succession of single-layer coatings of TiC, TiN or HfN were offered. The introduction of multilayer coatings brought about a further increase in wear resistance. These coatings either consist of TiC or Ti(C,N) with a thin surface layer of Al.sub.2 O.sub.3 or alternatively comprise a succession of layers made up of TiC adjoining the base material, then a series of titanium carbonitrides and finally a surface layer of TiN.
Since TiC has a higher hardness than TiN, 2900 Kg/mm.sup.2 and 2000 Kg/mm.sup.2 respectively, TiC coated tools tend to have higher flank wear resistance than TiN coated tools. However, TiN has a lower free energy of formaton than TiC, about 70 Kcal/gm-atm. and about 35 Kcal/gm-atm. respectively, and therefore a TiN coating is a more effective diffusion barrier and is more resistant to chemical wear, which causes crater wear, compared to a TiC coating.
U.S. Pat. Nos. 4,101,703 and 4,162,338 (Schintlmeister) describe coated cemented carbide cutting elements wherein the coating is a multilayer composition of at least two different wear resistant materials and includes at least two elements of the group consisting of carbon, nitrogen, boron and silicon in chemical combination with titanium. The coated materials are reported to exhibit superior wear resistance, especially in use for cutting steel and/or cast iron.
U.S. Pat. No. 4,035,541 (Smith et al.) describes a triple coated cemented metal carbide product. The triple coating is created by taking the metal carbide substrate material and (1) putting a first coating on the substrate, which comprises a carbide of a metal selected from the group of titanium, zirconium, hafnium, vanadium, niobium or tantalum; (2) putting a second coating on top of the first coating, the second coating comprising a metal carbonitride; and (3) putting a third and final coating over the second coating, the third coating comprising a metal nitride coating. It is reported that this arrangement enhances the hardness and wear life of the cemented metal carbide substrate material.
U.K. Pat. No. 1,601,224 (Lardner et al.) describes a cutting insert material provided with a first thin layer of titanium nitride, followed by a thin layer of titanium carbide, followed by an outer layer of titanium nitride or titanium carbonitride, having a thickness of up to 3 .mu.m.